1. Field of the Invention
The invention relates generally to the field of three dimensional audio technology and more particularly to the use of head related transfer functions (HRTF) for separating and imposing spatial cues to a plurality of audio signals in order to generate local virtual sources thereof such that each incoming signal is heard at a different location about the head of a listener.
2. Description of the Prior Art
Three dimensional or simply 3-D audio technology is a generic term associated with a number of new systems that have recently made the transition from the laboratory to the commercial audio world. Many of the terms have been used both commercially and technically to describe this technique, such as, dummy head synthesis, spatial sound processing, etc. All these techniques are related in their desired result of providing a psychoacoustically enhanced auditory display.
Much in the same way that stereophonic and quadraphonic signal processing devices have been introduced in the past as improvements over their immediate predecessors, 3-D audio technology can be considered as the most recent innovation for both mixing consoles and reverberation devices.
Three dimensional audio technology utilizes the concept of digital filtering based on head related transfer functions (HRTF). The role of the HRTF was first summarized by Jens Blauert in "Spatial Hearing: the psychophysics of human sound localization" MIT Press, Cambridge, 1983. This publication noted that the pinnae of the human ears are shaped to provide a transfer function for received audio signals and thus have a characteristic frequency and phase response for a given angle of incidence of a source to a listener. This characteristic response is convolved with sound that enters the ear and contributes substantially to our ability to listen spatially.
Accordingly, this spectral modification imposed by an HRTF on an incoming sound has been established as an important cue for auditoryspatial perception, along with interaural level and amplitude differences. The HRTF imposes a unique frequency response for a given sound source position outside of the head, which can be measured by recording the impulse response in or at the entrance of the ear canal and then examining its frequency response via Fourier analysis. This binaural impulse response can be digitally implemented in a 3-D audio system by convolving the input signal in the time domain with the impulse response of two HRTFs, one for each ear, using two finite impulse response filters. This concept was taught, for example, in 1990 by D. R. Begault et al in "Technical Aspects of a Demonstration Tape for Three-Dimensional Sound Displays" (TM 102826), NASA--Ames Research Center and also in U.S. Pat. No. 5,173,944, "Head Related Transfer Function Pseudo-Stereophony", D. R. Begault, Dec. 22, 1992.
The primary application of 3-D sound, however, has been made towards the field of entertainment and not towards improving audio communications systems involving intelligibility of multiple streams of speech in a noisy environment. Thus the focus of recent research and development for 3-D audio technology has centered on either commercial music recording, playback and playback enhancement techniques or on utilizing the technology in advanced human-machine interfaces such as computer work stations, aeronautics and virtual reality systems. The following cited literature is typically illustrative of such developments: D. Griesinger, (1989), "Equalization and Spatial Equalization of Dummy Head Recordings or Loudspeaker Reproduction", Journal of Audio Engineering Society, 37 (1-2), 20-29; L. F. Ludwig et al (1990), "Extending the Notion of a Window System To Audio", Computer, 23 (8), 66-72; D. R. Begault et al (1990), "Techniques and Application For Binaural Sound Manipulation in Human-Machine Interfaces" (TM102279), NASA-Ames Research Center; and E. M. Wenzel et al (1990), "A System for Three-Dimensional Acoustic Visualization in a Virtual Environment Work Station", Visualization '90, IEEE Computer Society Press, San Francisco, Calif. (pp. 329-337).
The following patented art is also directed to 3-D audio technology and is worthy of note: U.S. Pat. No. 4,817,149, "Three Dimensional Auditory Display Apparatus And Method Utilizing Enhanced Bionic Emulation Of Human Binaural Sound Localization", Peter H. Meyers, Mar. 28, 1989; U.S. Pat. No. 4,856,064, "Sound Field Control Apparatus", M. Iwamatsu, Aug. 8, 1989; and U.S. Pat. No. 4,774,515, "Attitude Indicator", B. Gehring, Sep. 27, 1988. The systems disclosed in these references simulate virtual source positions for audio inputs either with speakers, e.g. U.S. Pat. No. 4,856,064 or with headphones connected to magnetic tracking devices, e.g. U.S. Pat. No. 4,774,515 such that the virtual position of the auditory source is independent of head movement.